Method and apparatus for demodulating and equalizing an AM compatible digital audio broadcast signal

ABSTRACT

A method of demodulation and equalization is used to process an amplitude modulated compatible digital broadcasting signal including an amplitude modulated radio frequency signal having a first carrier modulated by an analog program signal in a first frequency spectrum, a plurality of digitally modulated carrier signals positioned within a bandwidth which encompasses the first frequency spectrum, a first group of the digitally modulated carrier signals including complementary carrier signals and lying within the first frequency spectrum, and second and third groups of the digitally modulated carrier signals including non-complementary carrier signals and lying outside of the first frequency spectrum. The method includes the steps of: taking a first Fast Fourier Transform of the amplitude modulated compatible digital signal to produce a first transformed signal representative of the non-complementary carriers; processing the first transformed signal to produce a first equalized signal by multiplying the first transformed signal by a first equalization vector, wherein the first equalization vector includes a first plurality of equalizer coefficients; updating the first plurality of equalizer coefficients; taking a second Fast Fourier Transform of the amplitude modulated compatible digital signal to produce a second transformed signal representative of the complementary carriers; determining a second equalization vector comprising a second plurality of equalizer coefficients, wherein the second plurality of equalizer coefficients is determined by interpolating coefficients of the first plurality of equalizer coefficients; and processing the second transformed signal to produce a second equalized signal by multiplying the second transformed signal by the second equalization vector. The invention also encompasses the operation of radio frequency receivers which utilize the above method, as well as apparatus that performs the above method and radio frequency receivers which utilize the above equalization method.

BACKGROUND OF THE INVENTION

[0001] This invention relates to radio broadcasting and, moreparticularly, to methods of and apparatus for demodulating andequalizing a signal in a receiver for an amplitude modulated compatibledigital broadcasting system.

[0002] There has been increasing interest in the possibility ofbroadcasting digitally encoded audio signals to provide improved audiofidelity. Several approaches have been suggested. One such approach, setforth in U.S. Pat. No. 5,588,022, teaches a method for simultaneouslybroadcasting analog and digital signals in a standard AM broadcastingchannel. The broadcast signal includes an amplitude modulated radiofrequency signal having a first frequency spectrum. The amplitudemodulated radio frequency signal includes a first carrier modulated byan analog program signal. The signal also includes a plurality ofdigitally modulated carrier signals within a bandwidth which encompassesthe first frequency spectrum. Each of the digitally modulated carriersignals is modulated by a portion of a digital program signal. A firstgroup of the digitally modulated carrier signals lies within the firstfrequency spectrum and is modulated in quadrature with the first carriersignal. Second and third groups of the digitally modulated carriersignals lie outside of the first frequency spectrum and are modulatedboth in-phase and in-quadrature with the first carrier signal.

[0003] The waveform in the AM compatible digital audio broadcastingsystem described in U.S. Pat. No. 5,588,022, was formulated to provideoptimal data throughput for the digital signal while avoiding crosstalkinto the analog AM channel. Multiple carriers are employed by means oforthogonal frequency division multiplexing (OFDM) to bear thecommunicated information.

[0004] Monophonic detectors for consumer AM radios respond only to theenvelope and not the phase of the received signal. Because of the use ofthe multiple digitally modulated carriers, there is a need for a meansto reduce the envelope distortion caused by this hybrid signal. U.S.patent application Ser. No. 08/671,252, assigned to the assignee of thepresent invention, discloses a method for reducing envelope distortionin an AM compatible digital audio broadcasting system. Certain digitalcarriers that are above the frequency of the analog AM carrier have anassociated digital carrier that is at an equal frequency offset belowthe analog AM carrier. The data and modulation placed on the upperdigital carrier and its counterpart are such that the signal resultingfrom their addition has no component that is in-phase with the analog AMcarrier. Digital carrier pairs arranged in this way are said to becomplementary. Carriers that are not directly beneath the analog signalspectrum are called non-complementary, and are modulated in-phase andin-quadrature to the AM carrier. This configuration delivers dramaticfidelity improvements to analog AM reception of digital broadcastsignals.

[0005] At the receiver, the digital signal is demodulated by means of aFast Fourier Transform (FFT). One possible method and associatedapparatus to perform the demodulation is described in U.S. Pat. No.5,633,896. That patent discloses a demodulation technique whichminimizes the undesired crosstalk between the analog signal and thedigital signals in an AM compatible digital audio broadcasting (AM DAB)system using an orthogonal frequency division multiplexed (OFDM)modulation format, by employing dual Fast Fourier Transform processes onseparate respective in-phase and quadrature-phase components of areceived OFDM digital signal. The output of the quadrature channel isused to recover the complementary data, and the resultant processedcomponent signals are summed to recover the non-complementary data.

[0006] The received multi-carrier signal requires equalization in thepresence of dynamic channel response variations. Without suchequalization, a very distorted signal would be detected and the digitalbroadcasting signal information would be unrecoverable. An equalizerenhances the recoverability of the digital audio broadcasting signalinformation. One such equalizer is disclosed in U.S. Pat. No. 5,559,830.The equalizer disclosed therein includes means for receiving an AMcompatible digital audio broadcasting waveform and storing that waveformas a waveform vector. The equalizer then processes that waveform bymultiplying the waveform vector by an equalization vector. Thisequalization vector comprises a plurality of equalizer coefficients,each of the coefficients initially set to a predetermined value. Theequalizer then compares each location of the processed waveform vectorwith a stored waveform vector. The equalizer selects as the signal thatvector location closest to the stored waveform vector. Preferably, theequalizer includes means for updating the equalizer coefficients usingthe waveform vector, the processed waveform vector, and the storedwaveform vector to provide immunity to noise and response to channelchanges.

[0007] In the equalizers of both U.S. Pat. Nos. 5,633,896 and 5,559,830,frequency domain information is presented to the equalizer as afrequency domain vector. Each block of frequency domain information isstored in a storage array. This storage array vector is multiplied by aplurality of equalizer coefficients. The resulting product of thismultiplication is an equalized signal. A set of exact values is known apriori in the equalizer against which each vector location of theequalized signal can be compared. The ideal value closest to thatdescribed in the vector location is chosen as the actual signal value.The vector of decisions is stored in a decision array. Using thereceived signal, the equalized signal, and the decision array, anequalizer coefficient estimator calculates coefficient estimates. Toprovide immunity to noise, the equalizer coefficient estimates can beaveraged over time. The rate of coefficient update determines equalizernoise immunity and convergence rate. Coefficients in different parts ofthe band may be updated at different rates depending on knowledge of thedistortion mechanism. U.S. Pat. Nos. 5,633,896 and 5,559,830 are herebyincorporated by reference.

[0008] While the dual FFT technique can improve system performance in achannel that has symmetric magnitude and anti-symmetric phase about theAM carrier frequency over the frequency range of the complementarycarriers, for channels with non-symmetric magnitude ornon-anti-symmetric phase, the process of using only the quadraturechannel FFT output to obtain the complementary data destroys thenon-symmetric magnitude and non-anti-symmetric phase information and thesignal that drives the equalizer is not correct. A United States PatentApplication for a “Method For Equalization Of Complementary Carriers InAn AM Compatible Digital Audio Broadcast System”, by the presentinventors, filed on the same date as this application, and assigned tothe same assignee, discloses ah equalization method that can provideproper equalizer coefficients when the equalizer coefficients may havenon-symmetric magnitude or non-anti-symmetric phase.

[0009] Demodulation of the non-complementary carriers may require ahigh-pass filter on the in-phase portion of the signal to eliminatespectral spillage in the FFT from the analog signal. However, when ahigh-pass filter is applied, information in the in-phase signal isdestroyed, thus preventing proper equalization of the complementarydigital carriers. For channels that have non-symmetric magnitude ornon-anti-symmetric phase over the spectral region of the analog signal,the destroyed information prevents proper equalization of thecomplementary carriers. The channel, as referred to here, includes notonly phenomenon that affect propagation of the signal, but also anycomponent in the transmitter or receiver that affects the magnitude andphase of the received signal. The present invention provides a method ofdemodulating the digital signal without the drawback of either spectralspillage of the analog signal onto the non-complementary carriers ordestroying information needed for proper equalization of thecomplementary carriers. The present invention seeks to provide animproved demodulation and equalization method and receivers whichinclude the method.

SUMMARY OF THE INVENTION

[0010] The present invention provides a method of demodulating andequalizing an AM compatible digital broadcast signal. The methodincludes estimating the equalizer coefficients for the complementarycarriers while still retaining the benefits of combining the informationfrom the complementary carrier FFT outputs. The method uses informationfrom the non-complementary carriers to estimate, via interpolation, theequalizer coefficients for the complementary carriers.

[0011] The demodulation and equalization method of the present inventionis used to process an amplitude modulated compatible digitalbroadcasting signal including an amplitude modulated radio frequencysignal having a first carrier modulated by an analog program signal in afirst frequency spectrum, a plurality of digitally modulated carriersignals positioned within a bandwidth which encompasses the firstfrequency spectrum, a first group of the digitally modulated carriersignals including complementary carrier signals and lying within thefirst frequency spectrum, and second and third groups of the digitallymodulated carrier signals including non-complementary carrier signalsand lying outside of the first frequency spectrum. The method includesthe steps of: taking a first Fast Fourier Transform of the amplitudemodulated compatible digital signal to produce a first transformedsignal representative of the non-complementary carriers; processing thefirst transformed signal to produce a first equalized signal bymultiplying the first transformed signal by a first equalization vector,wherein the first equalization vector includes a first plurality ofequalizer coefficients; updating the first plurality of equalizercoefficients; taking a second Fast Fourier Transform of the amplitudemodulated compatible digital signal to produce a second transformedsignal representative of the complementary carriers; determining asecond equalization vector comprising a second plurality of equalizercoefficients, wherein the second plurality of equalizer coefficients isdetermined by interpolating coefficients of the first plurality ofequalizer coefficients; and processing the second transformed signal toproduce a second equalized signal by multiplying the second transformedsignal by the second equalization vector.

[0012] The invention also encompasses the operation of radio frequencyreceivers which utilize the above method, as well as apparatus thatperforms the above method and radio frequency receivers which utilizethe above equalization method.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The invention will be more readily apparent to those skilled inthe art by reference to the accompanying drawing wherein:

[0014]FIG. 1 is a diagrammatic representation of a prior art compositeanalog AM and digital broadcasting signal;

[0015]FIG. 2 is a block diagram of a receiver which includes anequalizer that operates in accordance with this invention;

[0016]FIG. 3 is a functional block diagram of a demodulator and adaptiveequalizer in accordance with this invention;

[0017]FIGS. 4a and 4 b are phasor diagrams which illustrate theoperation of the invention; and

[0018]FIG. 5 is a diagram showing the magnitude of the response of theequalizer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] This invention provides a method for demodulating and equalizingcarriers in a broadcast signal which includes both an analog amplitudemodulated signal and a digital signal on the same channel assignment asthe existing analog AM broadcasting allocation. The technique ofbroadcasting the digital signal in the same channel as an analog AMsignal is called in-band on-channel (IBOC) broadcasting. Thisbroadcasting is accomplished by transmitting a digital waveform by wayof a plurality of orthogonal frequency division modulated (OFDM)carriers, some of which are modulated in-quadrature with the analog AMsignal and are positioned within the spectral region where the analog AMbroadcasting signal has significant energy. The remaining digitalcarriers are modulated both in-phase and in-quadrature with the analogAM signal and are positioned in the same channel as the analog AMsignal, but in spectral regions where the analog AM signal does not havesignificant energy. In the United States, the emissions of AMbroadcasting stations are restricted in accordance with FederalCommunications Commission (FCC) regulations to lie within a signal levelmask defined such that: emissions 10.2 kHz to 20 kHz removed from theanalog carrier must be attenuated at least 25 dB below the unmodulatedanalog carrier level, emissions 20 kHz to 30 kHz removed from the analogcarrier must be attenuated at least 35 dB below the unmodulated analogcarrier level, and emissions 30 kHz to 60 kHz removed from the analogcarrier must be attenuated at least [35 dB+1 dB/kHz] below theunmodulated analog carrier level.

[0020]FIG. 1 shows the spectrum of an AM digital audio broadcastingsignal of a type which can be utilized by the present invention. Curve10 represents the standard broadcasting amplitude modulated signal,wherein the carrier has a frequency of f₀. The FCC emissions mask isrepresented by item number 12. The OFDM waveform is composed of a seriesof data carriers spaced at f₁=59.535·10⁶/(131072), or about 454 Hz. Afirst group of twenty four of the digitally modulated carriers arepositioned within a frequency band extending from (f₀−12 f₁) to (f₀+12f₁), as illustrated by the envelope labeled 14 in FIG. 1, Most of thesesignals are placed 39.4 dB lower than the level of the unmodulated AMcarrier signal in order to minimize crosstalk with the analog AM signal.Crosstalk is further reduced by encoding this digital information in amanner that guarantees orthogonality with the analog AM waveform. Thistype of encoding is called complementary encoding (i.e. complementaryBPSK, complementary QPSK, or complementary 32 QAM) and is more fullydescribed in the previously discussed copending application Ser. No.08/671,252. Complementary BPSK modulation is employed on the innermostdigital carrier pair at f₀±f₁ to facilitate timing recovery. Thesecarriers are set at a level of −28 dBc. All other carriers in this firstgroup have a level of −39.4 dBc and are modulated using complementary 32QAM for the 48 and 32 kbps encoding rates. Complementary 8 PSKmodulation is used on carriers ranging from (f₀−11 f₁) to (f₀−2 f₁) andfrom (f₀+2 f₁) to (f₀+11 f₁) for the 16 kbps coding rate. For all threeencoding rates, the carriers at (f₀−12 f₁) and (f₀+12 f₁) carrysupplementary data and may be modulated using complementary 32 QAM.

[0021] Additional groups of digital signals are placed outside the firstgroup. The need for these digital waveforms to be in-quadrature with theanalog signal is eliminated by restricting the analog AM signalbandwidth. The carriers in a second and a third group, encompassed byenvelopes 16 and 18 respectively, may be modulated using, for example,32 QAM for the 48 and 32 kbps rates, and 8 PSK for the 16 kbps rate. Thecarriers are set at levels of −30 dBc for all encoding rates.

[0022]FIG. 2 is a block diagram of a receiver constructed to receive thecomposite digital and analog signals of FIG. 1. An antenna 110 receivesthe composite waveform containing the digital and analog signals andpasses the signal to conventional input stages 112, which may include aradio frequency preselector, an amplifier, a mixer and a localoscillator. An intermediate frequency signal is produced by the inputstages on line 114. This intermediate frequency signal is passed throughan automatic gain control circuit 116 to an I/Q signal generator 118.The I/Q signal generator produces an in-phase signal on line 120 and aquadrature signal on line 122. The in-phase channel output on line 120is input to an analog-to-digital converter 124. Similarly, thequadrature channel output on line 122 is input to anotheranalog-to-digital converter 126. Feedback signals on lines 120 and 122are used to control the automatic gain control circuit 116. The signalon line 120 includes the analog AM signal which is separated out asillustrated by block 140 and passed to an output stage 142 andsubsequently to a speaker 144 or other output device.

[0023] A demodulator 150 receives the digital signals on lines 128 and130, and produces output signals on lines 154. These output signals arepassed to an equalizer 156 and to a data rate filter and data decoder158. The output of the data decoder is sent to a deinterleaving circuitand forward error correction decoder 164 in order to improve dataintegrity. The output of the deinterleaver/forward error correctingcircuit is passed to a source decoder 166. The output of the sourcedecoder is delayed by circuit 168 to compensate for the delay of theanalog signal at the transmitter and to time align the analog anddigital signals at the receiver. The output of delay circuit 168 isconverted to an analog signal by a digital-to-analog converter 160 toproduce a signal on 162 which goes to the output stage 142.

[0024] U.S. Pat. No. 5,559,830, issued Sep. 24, 1996 describes one modeof operation for an equalizer having an equalizer coefficient updatealgorithm. The present invention enhances the operation of the equalizerand equalizer coefficient update algorithm by considering the effectsthat occur when the equalizer coefficients should have non-symmetricmagnitude or non-anti-symmetric phase about the center of the FFT.

[0025]FIG. 3 is a functional block diagram of a portion of the receiverprocessing which illustrates the operation of the present invention.Both in-phase (I) and quadrature (Q) signals are provided on lines 128and 130. These signals may be provided by using down converter elementssimilar to those shown in FIG. 2. To eliminate the analog signal priorto being input to a first Fast Fourier Transform processor (FFT1) inblock 170, a high pass filter 174 has been added to filter the in-phasecomponents of the signal on line 128 thereby producing a filtered signalon line 148. Signals on lines 148 and 130 are processed by windowing andguard band removal circuit 171 prior to being input to FFT1. The windowshould be applied such that the digital carriers remain orthogonal, orat least the lack of orthogonality among the digital carriers is smallenough not to impact system performance. A method of applying a windowthat preserves orthogonality among the carriers has been developed. In aspecific implementation of the method, a root-raised cosine window isapplied at the transmitter and receiver. For this window, the taperingoccurs on the first and last seven samples of the 135 samples in a baud.After the window has been applied at the receiver, the last sevensamples are added to the first seven samples, where the 129th sample isadded to the first sample, the 130th sample is added to the secondsample, and this pattern continues with the 135th sample being added tothe seventh sample. The resulting 128 points are input to an FFT. Insome cases it may be advantageous to perform the windowing and guardband removal operations prior to processing by highpass filter 174. Inthis case, the windowing and guard band removal operations performed bycircuits 171 and 173 could be combined to be performed by one circuit.

[0026] Elimination of the analog signal may be necessary to preventspectral spillage from the analog signal onto the in-phase portion ofthe non-complementary carriers. The disadvantage of this highpass filteris that information needed to properly equalize and demodulate thecomplementary carriers can be destroyed when the channel has anon-symmetric magnitude or non-anti-symmetric phase about the analog AMcarrier frequency. If the in-phase input to FFT1 is high pass filteredto eliminate the analog signal, the output of FFT1, which is input tothe equalizer coefficient update algorithm, has certain symmetryproperties. Specifically, since the in-phase part of the FFT1 input hasnearly zero energy for the complementary carriers, the output of FFT1will have nearly anti-hermitian symmetry for the complementary carriers.The output of the symbol decision processor for the complementarycarriers will have the same property. Since these two anti-hermitiansignals serve as the input to the equalizer coefficient update routine,the equalizer coefficients will be constrained to have a magnituderesponse that is symmetric and a phase response that is anti-symmetricabout the center frequency of FFT1. Therefore, the equalizercoefficients will not converge to the proper values when the equalizercoefficients should have non-symmetric magnitude or non-anti-symmetricphase about the center of FFT1.

[0027] The outputs of FFT1 that correspond to the non-complementarycarriers are input by way of lines 176 to a first equalizer 178.Equalizer 178 operates on the frequency domain data and adjusts themagnitude and phase of each OFDM carrier to counteract the effects ofchannel perturbations, transmitter and receiver filters, the transmitand receive antennas, and other factors and processing that affect themagnitude and phase of the signal. The outputs of equalizer 178 on lines180 are fed to a symbol decision processor 182 which produces signals onlines 184 that are representative of the digital information carried bythe non-complementary carriers of the AM compatible broadcast waveform.

[0028] Information on lines 176 and 184 is used to update thecoefficients of the equalization coefficient vectors in equalizer EQ1,as illustrated by block 186. The coefficients to be applied to thecomplementary carriers that are processed by equalizer EQ2 in block 188,are determined by interpolation as illustrated in block 190. The inputsignals 128 and 130 are processed by windowing and guard intervalcircuit 173 and then input to Fast Fourier Transform processor FFT2,which produces outputs corresponding to the complementary carriers andprovides these outputs as inputs to equalizer EQ2 on lines 192. Theoutput of equalizer 156 of FIG. 2 can consist of the combination of theoutputs of EQ1 178 and EQ2 188 in FIG. 3, or the combination of signals184 and 202 in FIG. 3, depending on the type of data required forfurther processing, which may especially depend on the type of forwarderror correction (FEC) used in the system. If symbol decision outputsare required, higher signal-to-noise ratios (SNR) for the complementarycarriers can be obtained by combining the FFT outputs for pairs ofcomplementary carriers. Specifically, the data from one complementarycarrier is added to the negative conjugate of the other complementarycarrier and the average is calculated. For each pair of complementarycarriers processed by equalizer EQ2, block 194 shows that the negativeconjugate of one carrier in the pair is added to the other carrier inthe pair as illustrated by adders 196 and 198. Symbol processor 200 thenproduces outputs that are representative of the digital informationcarried by the complementary carriers of the AM compatible broadcastsignal.

[0029]FIGS. 4a and 4 b are vector diagrams which can be used to furtherillustrate the operation of the invention. FIG. 4a shows a phasordiagram of the transmitted signal The horizontal axis is the I componentand the vertical axis is the Q component. The constant AM carrier levelis shown as the phasor 204 along the horizontal axis, and the phasordiagram is fixed with respect to the frequency of the AM carrier. Alsoshown in FIG. 4a are two AM sideband signals 206 and 208. These signalsrepresent an analog tone. Note that FIG. 4a shows the resultant 210, orvector addition, of the analog sidebands. The resultant is on the Iaxis, and will continue to be on the I axis as the analog sidebandsrotate. FIG. 4a also shows the phasors 212 and 214 for one pair ofcomplementary carriers. The resultant 216 of these carriers is on the Qaxis and stays on the Q axis as the complementary carriers rotate.

[0030]FIG. 4b shows the phasor diagram at the receiver assuming achannel that is non-symmetric in magnitude and non-anti-symmetric inphase. As can be seen, now the resultant 216′ of the complementarycarrier pair 212′ and 214′ has energy in both the I and Q signals. Ifthe I signal at the frequency of the complementary carrier pair iseliminated by the highpass filter shown in FIG. 3, the signal cannot beproperly equalized and demodulated. Although FIGS. 4a and 4 b show onlyone complementary carrier pair, the above statements apply to all of thecomplementary carriers. FIGS. 4a and 4 b show another effect thatprevents proper demodulation of the complementary carriers. Theresultant 210′ of the analog sidebands 206′ and 208′ also has energy inboth the I and Q signals. Therefore, because some of the analog signalenergy is now in the Q signal, this also prevents proper demodulation ofthe complementary carriers. Therefore, the output of FFT1 cannot be usedto properly demodulate the complementary carriers when the channel isnon-symmetric in magnitude and non-anti-symmetric in phase. However, theoutput of FFT1 can be used to equalize and demodulate thenon-complementary carriers. Because only the non-complementaryinformation is used at the output of FFT1, only the outputs for thenon-complementary carriers need to be calculated. As shown in FIG. 3,the output of FFT1 is input to a first equalizer, denoted as EQ1. Thisequalizer, as well as a second equalizer denoted by EQ2, operate on thefrequency domain data and adjust the magnitude and phase of the OFDMcarriers to counteract the effects of propagation channel perturbations,transmitter and receiver filters, transmit and receive antennas, andother factors and processing that affect the magnitude and phase of thereceived signal. The output of EQ1 is input to a symbol decisionprocessor that determines which of the frequency domain constellationpoints was transmitted for each non-complementary carrier. Thesedecisions, along with the pre-equalized constellation points and theprevious values of the equalizer coefficients are used to update theequalizer coefficients. An algorithm such as the least mean squares(LMS) or recursive least squares (RLS) can be used to update theequalizer coefficients.

[0031] As shown in FIG. 3, FFT2 is used to obtain the information forthe complementary carriers. The I signal that is input to FFT2 is nothighpass filtered, and therefore all of the information needed toequalize and demodulate the complementary carriers is available at theoutput of FFT2. Because only the complementary information is used atthe output of FFT2, only the outputs for the complementary carriers needto be calculated. The output of FFT2 is equalized by EQ2. As shown inFIG. 3, for each complementary carrier pair, the negative conjugate ofone carrier in the pair is added to the other carrier in the pair. Thesum is then used to make a symbol decision for the complementary pair.The coefficients for EQ2 could be updated in the same manner as thecoefficients for EQ1, but the presence of the analog signal would makethe coefficient estimates noisy. To overcome this, the equalizercoefficients for EQ2 can be obtained via interpolation using thecoefficients for EQ1. If the control loops of the receiver such as theautomatic gain control (AGC), carrier tracking, and symbol tracking areat the proper values, the center frequency of the FFT will be at aknown, constant magnitude and phase.

[0032]FIG. 5 illustrates an example where linear interpolation is usedto determine the equalizer coefficients across the complementarycarriers. FIG. 5 actually shows the inverse of the channel response 218because this is the desired response for the equalizer. The response 220that would be obtained from the equalizer magnitude is also shown inFIG. 5. For clarity, the illustrated equalizer response is displacedupward slightly so it can be distinguished from the inverse channelresponse. Note that the response follows the inverse channel response inthe regions 222 and 224 of the non-complementary carriers. As can beseen, if the channel response 218 is relatively smooth, the interpolatedequalizer coefficients are near to the ideal values, and the equalizermagnitude response 220 closely follows the inverse channel magnitude inthe region 226 of the complementary carriers.

[0033] Several variations of interpolation are possible. For example,the value of the equalizer coefficient for the first OFDM carriersoutside of the complementary region could be used to linearlyinterpolate from their values to the value at the center of the channel.Linear interpolation has been found to be satisfactory in the largemajority of cases where the signal is in the commercial AM broadcastband (530 kHz to 1710 kHz) and the width of the complementary region isless than 10 kHz. As an alternative, it may be desirable to usenon-complementary carriers that are further away from the center of thechannel if the non-complementary carrier or carriers that are locatedclosest to the complementary carrier region are affected by filters suchas the highpass filter that can be used to eliminate the analog signalfrom the in-phase portion of the received signal. Also, information frommany of the non-complementary carriers could be used in theinterpolation process. Interpolation algorithms other than linear couldbe used. Some of the well known interpolation algorithms include cubicspline, polynomial interpolation, FFT based interpolation, andexponential or logarithmic curve fitting. The non-complementaryequalizer coefficients used for the interpolation and the complementaryequalizer coefficients obtained from the interpolation can be averagedover time to reduce the effects of noise. Smoothing across frequency canalso be used to reduce the effects of noise. Instead of interpolatingthe linear magnitude of the coefficients, interpolation on a logmagnitude scale may be advantageous. Alternatively, instead ofinterpolating the magnitude and phase of the equalizer coefficients, itmay be desirable to interpolate the corresponding real and imaginarycoordinates that can be used to represent the equalizer coefficients.

[0034] This invention provides a system for demodulating and adaptivelyequalizing an amplitude modulated compatible digital audio broadcastsignal. In the foregoing specification certain preferred practices andembodiments of this invention have been set out, however, it will beunderstood that the invention may be otherwise embodied within the scopeof the following claims.

1. A method of demodulating and equalizing an amplitude modulatedcompatible digital broadcasting signal including an amplitude modulatedradio frequency signal having a first carrier modulated by an analogprogram signal in a first frequency spectrum, a plurality of digitallymodulated carrier signals positioned within a bandwidth whichencompasses the first frequency spectrum, a first group of the digitallymodulated carrier signals including complementary carrier signals andlying within the first frequency spectrum, and second and third groupsof the digitally modulated carrier signals including non-complementarycarrier signals and lying outside of the first frequency spectrum, saidmethod comprising the steps of: taking the Fast Fourier Transform of theamplitude modulated compatible digital broadcasting signal to produce afirst transformed signal representative of the non-complementarycarriers; processing said first transformed signal to produce a firstequalized signal by multiplying said first transformed signal by a firstequalization vector, said first equalization vector comprising a firstplurality of equalizer coefficients; updating said first plurality ofequalizer coefficients used for the non-complementary signals; takingthe Fast Fourier Transform of the amplitude modulated compatible digitalbroadcasting signal to produce a second transformed signalrepresentative of the complementary carriers; determining a secondequalization vector comprising a second plurality of equalizercoefficients, said second plurality of equalizer coefficients beingdetermined by interpolation using coefficients of said first pluralityof equalizer coefficients; and processing said second transformed signalto produce a second equalized signal by multiplying said secondtransformed signal by said second equalization vector.
 2. The method ofclaim 1, further comprising the steps of: separating the amplitudemodulated compatible digital broadcasting signal into in-phase andquadrature components; and filtering the amplitude modulated compatibledigital broadcasting signal in phase component prior to the step oftaking the Fast Fourier Transform of the amplitude modulated compatibledigital broadcasting signal to produce said first transformed signal. 3.The method of claim 2, wherein the step of filtering the in-phasecomponent comprises: passing the in-phase component through a highpassfilter.
 4. The method of claim 1, further comprising the step of:windowing the amplitude modulated compatible digital broadcast signaland removing a guard interval from the amplitude modulated compatibledigital broadcast signal prior to each of the steps of taking the FastFourier Transform of the amplitude modulated compatible digitalbroadcast signal.
 5. The method of claim 1, wherein said secondplurality of equalizer coefficients are calculated using said first setof equalizer coefficients and a known value at the center of the firstfrequency spectrum, such calculation being performed by interpolationusing one of linear interpolation, cubic spline interpolation,polynomial interpolation, Fast Fourier Transform based interpolation, orlogarithmic curve fitting.
 6. The method of claim 1, wherein saidinterpolation is averaged over time.
 7. The method of claim 1, whereinsaid interpolation is performed on real and imaginary components used torepresent said first and second plurality of equalizer coefficients. 8.A method of operating a radio frequency receiver for receiving anamplitude modulated compatible digital broadcasting signal including anamplitude modulated radio frequency signal having a first carriermodulated by an analog program signal in a first frequency spectrum, aplurality of digitally modulated carrier signals positioned within abandwidth which encompasses the first frequency spectrum, a first groupof the digitally modulated carrier signals including complementarycarrier signals and lying within the first frequency spectrum, andsecond and third groups of the digitally modulated carrier signalsincluding non-complementary carrier signals and lying outside of thefirst frequency spectrum, said method comprising the steps of: receivingsaid amplitude modulated compatible digital broadcasting signal; takingthe Fast Fourier Transform of said amplitude modulated compatibledigital broadcasting signal to produce a first transformed signalrepresentative of the non-complementary carriers; processing said firsttransformed signal to produce a first equalized signal by multiplyingsaid first transformed signal by a first equalization vector, said firstequalization vector comprising a first plurality of equalizercoefficients; updating said first plurality of equalizer coefficientsused for the non-complementary signals; taking the Fast FourierTransform of said amplitude modulated compatible digital broadcastingsignal to produce a second transformed signal representative of thecomplementary carriers; determining a second equalization vectorcomprising a second plurality of equalizer coefficients, said secondplurality of equalizer coefficients being determined by interpolationusing coefficients of said first plurality of equalizer coefficients;processing said second transformed signal to produce a second equalizedsignal by multiplying said second transformed signal by said secondequalization vector; and producing an output signal in response to saidfirst and second equalized signals.
 9. The method of claim 8, furthercomprising the steps of: separating said amplitude modulated compatibledigital broadcasting signal into in-phase and quadrature components; andfiltering the amplitude modulated compatible digital broadcasting signalin-phase component prior to the step of taking the Fast FourierTransform of the amplitude modulated compatible digital broadcastingsignal to produce said first transformed signal.
 10. The method of claim9, wherein the step of filtering the in-phase component comprises:passing the in-phase component through a highpass filter.
 11. The methodof claim 8, further comprising the step of: windowing the amplitudemodulated compatible digital broadcast signal and removing a guardinterval from the amplitude modulated compatible digital broadcastsignal prior to each of the steps of taking the Fast Fourier Transformof the amplitude modulated compatible digital broadcast signal.
 12. Themethod of claim 8, wherein said second plurality of equalizercoefficients are calculated using said first set of equalizercoefficients and a known value at the center of the first frequencyspectrum, such calculation being performed by interpolation using one oflinear interpolation, cubic spline interpolation, polynomialinterpolation, Fast Fourier Transform based interpolation, orlogarithmic curve fitting.
 13. The method of claim 8, wherein saidinterpolation is averaged over time.
 14. The method of claim 8, whereinsaid interpolation is performed on real and imaginary components used torepresent said first and second plurality of equalizer coefficients. 15.An apparatus for demodulating and equalizing an amplitude modulatedcompatible digital broadcasting signal including an amplitude modulatedradio frequency signal having a first carrier modulated by an analogprogram signal in a first frequency spectrum, a plurality of digitallymodulated carrier signals positioned within a bandwidth whichencompasses the first frequency spectrum, a first group of the digitallymodulated carrier signals including complementary carrier signals andlying within the first frequency spectrum, and second and third groupsof the digitally modulated carrier signals including non-complementarycarrier signals and lying outside of the first frequency spectrum, saidmethod comprising the steps of: means for taking the Fast FourierTransform of the amplitude modulated compatible digital broadcastingsignal to produce a first transformed signal representative of thenon-complementary carriers; means for processing said first transformedsignal to produce a first equalized signal by multiplying said firsttransformed signal by a first equalization vector, said firstequalization vector comprising a first plurality of equalizercoefficients; means for updating said first plurality of equalizercoefficients used for the non-complementary signals; means for takingthe Fast Fourier Transform of the amplitude modulated compatible digitalbroadcasting signal to produce a second transformed signalrepresentative of the complementary carriers; means for determining asecond equalization vector comprising a second plurality of equalizercoefficients, said second plurality of equalizer coefficients beingdetermined by interpolating coefficients of said first plurality ofequalizer coefficients; and means for processing said second transformedsignal to produce a second equalized signal by multiplying said secondtransformed signal by said second equalization vector.
 16. The apparatusof claim 15, further comprising: means for separating thenon-complementary carrier signals into in-phase and quadraturecomponents; and means for filtering the non-complementary carrier signalin-phase component.
 17. The apparatus of claim 16, wherein the means forfiltering comprises: a highpass filter.
 18. The apparatus of claim 15,further comprising: means for windowing the amplitude modulatedcompatible digital broadcast signal and removing a guard interval fromthe amplitude modulated compatible digital broadcast signal.
 19. Theapparatus of claim 15, wherein said second plurality of equalizercoefficients are calculated using said first set of equalizercoefficients and a known value at the center of the first frequencyspectrum, such calculation being performed by interpolation using one oflinear interpolation, cubic spline interpolation, polynomialinterpolation, Fast Fourier Transform based interpolation, orlogarithmic curve fitting.
 20. The apparatus of claim 15, wherein saidinterpolation is averaged over time.
 21. The apparatus of claim 15,wherein said interpolation is performed on real and imaginary componentsused to represent said first and second plurality of equalizercoefficients.
 22. A radio frequency receiver for receiving an amplitudemodulated compatible digital broadcasting signal including an amplitudemodulated radio frequency signal having a first carrier modulated by ananalog program signal in a first frequency spectrum, a plurality ofdigitally modulated carrier signals positioned within a bandwidth whichencompasses the first frequency spectrum, a first group of the digitallymodulated carrier signals including complementary carrier signals andlying within the first frequency spectrum, and second and third groupsof the digitally modulated carrier signals including non-complementarycarrier signals and lying outside of the first frequency spectrum, saidreceiver comprising: means for receiving said amplitude modulatedcompatible digital broadcasting signal; means for taking the FastFourier Transform of the amplitude modulated compatible digitalbroadcasting signal to produce a first transformed signal representativeof the non-complementary carriers; means for processing said firsttransformed signal to produce a first equalized signal by multiplyingsaid first transformed signal by a first equalization vector, said firstequalization vector comprising a first plurality of equalizercoefficients; means for updating said first plurality of equalizercoefficients used for the non complementary signals; means for takingthe Fast Fourier Transform of the amplitude modulated compatible digitalbroadcasting signal to produce a second transformed signalrepresentative of the complementary carriers; means for determining asecond equalization vector comprising a second plurality of equalizercoefficients, said second plurality of equalizer coefficients beingdetermined by interpolating coefficients of said first plurality ofequalizer coefficients; means for processing said second transformedsignal to produce a second equalized signal by multiplying said secondtransformed signal by said second equalization vector; and means forproducing an output signal in response to said first and secondequalized signals.
 23. The receiver of claim 22, further comprising:means for separating the non-complementary carrier signals into in-phaseand quadrature components; and means for filtering the non-complementarycarrier signal in-phase components.
 24. The receiver of claim 23,wherein means for filtering comprises: a highpass filter.
 25. Thereceiver of claim 23, further comprising: means for windowing theamplitude modulated compatible digital broadcast signal and removing aguard interval from the amplitude modulated compatible digital broadcastsignal.
 26. The receiver of claim 23, wherein said second plurality ofequalizer coefficients are calculated using said first set of equalizercoefficients and a known value at the center of the first frequencyspectrum, such calculation being performed by interpolation using one oflinear interpolation, cubic spline interpolation, polynomialinterpolation, Fast Fourier Transform based interpolation, orlogarithmic curve fitting.
 27. The receiver of claim 23, wherein saidinterpolation is averaged over time.
 28. The receiver of claim 23,wherein said interpolation is performed on real and imaginary componentsused to represent said first and second plurality of equalizercoefficients.